Processing light metal alloys in a conventional way involves one of the two well know processes, namely cold chamber or hot chamber die-casting methods. These processes use melting furnace to melt light alloy at superheated temperatures and than inject the molten metal into a re-usable mold. Recycled material is also re-melted in the same furnace. In the process of melting magnesium cover gas is used to prevent magnesium from evaporation and burning. The cover gas used is often SF6 Sulfur hexafluoride. A report by the US World Resources Institute reported that the global warming potential for SF6 is 23,900 relative to CO2. This means that 1 kg of SF6 in the atmosphere gives approximately same contribution to green house effect as 24 tonnes of CO2 per tonne of the magnesium smelted. This gas is the dominant greenhouse contributor for magnesium smelters and die-casters. The life time of SF6 in the atmosphere is estimated to be 3200 years. Due to these environmental problems, new processes that do not require potent SF6 gas are being searched for worldwide.
Feedstock is often contaminated with organic and inorganic inclusions coming from various contamination points in a life cycle of the feedstock. These inclusions are often introduced by the chip manufacturer unintentionally due to poor process quality control. Organic inclusions could be dust or lint, for example. Some of the contamination is sourced back to exposure to environment and handling from start of the chip manufacturing to end use location. Recycled magnesium chips are often too contaminated by the oil, water, wax, mold release etc. If used in a casting process, these chips would make poor quality parts unsuitable for demanding automotive industry.
During processing, these inclusions and foreign material end up in the part and are seen in the castings, by metallurgical evaluation, as voids or are often converted with help of high melt temperature into oxides with very high re-melt temperature. The water molecules, on the other hand, and entrapped air or other gasses from the air get attached to the highly stressed surface of the comminuted particle due to known physical principles mostly in acute curvature of the chip. This causes in-homogeneity in melt and subsequently affects part quality. The water affects processing of the magnesium (not exclusive to magnesium) by creating explosive conditions where water is cracked into O2 and H2. The hydrogen H2 could create explosive mixture and unsafe processing. Humidity is undesirable within the feedstock. Attached molecules of other elements or gasses like oxygen and nitrogen to the chips are also undesirable input to the casting process.
All current, environmentally friendly, light metal casting processes could benefit from fine feedstock that is clean, free of all contaminants, oxygen and nitrogen free. When this feedstock is then pre-heated to 150° C., preferably up to 200° C. or even up to 250° C., or even more preferably up to 400° C. (for magnesium) it can significantly improve part quality and metallurgical properties of the casting. All processes using granulated feedstock may benefit from the current invention by receiving down stream clean, conditioned and tempered feedstock that is scrubbed from all contaminants and dried from moisture as well as purged from inclusions of air contaminants like chemically unattached oxygen and nitrogen molecules. Heating metallic feedstock is not being currently practiced in industry. Heating uniformly shaped feedstock is a challenge, but heating chips of non-uniform and random shapes is very difficult with any conventional heating means.
There are number of apparatuses claiming successful treatment of granular feedstock, but it is not known to these inventors, any application where randomly shaped light metal alloy chips are successfully and uniformly heated in temperature range 100° C. to 460° C. Heating granular substances other than light metal alloys has been used in industry for a long time. One form of the device for treating particulate product is U.S. Pat. No. 6,367,165 B1 where a granular product for generic pharmaceutical application claims the benefit of the baffle plate to distribute air in the fluidized chamber. Substantially horizontal input of air is claimed as main feature of this apparatus. Another disclosure U.S. Pat. No. 4,967,688 to Yoshiro at al., discloses powder processing apparatus with a rotating air permeable blades used to apply thin coating to chemical powders, treatment of food products and ceramic powders by liquid to apply thin coat of film to the particles. Both above disclosures are batch type structures. No cleaning and scrubbing features are mentioned or claimed in these disclosures.
In U.S. Pat. No. 4,372,053 Anderson et al. relates to a method of drying particulate material within an enclosed chamber. Heating and cooling fluids are introduced in particular zones of the dryer to accomplish particular moisture content of the various grains, like corn prior to storage.
In another U.S. Pat. No. 4,346,054 a fluidized bed apparatus is used for temperatures >700° C. processing iron ore where fluidizing medium is under high pressure and is CO2, N2 and or air. This also is a batch type fluidization apparatus. There seems to be a plethora of applications of the fluidized bed for efficient burning of organic matter and extracting heat from the burning medium. The U.S. Pat. No. 6,139,805 issued to Shuichi at al. using solid material containing a combustible and non-combustible material. Heat energy extracting plates are within fluidized bedchamber in a singular or multiple arrangements.
None of the prior art describes the apparatus for heating metallic particles in a continuous process where fluidization and energy input transfer is done by the same fluidizing medium and/or by the recycled heat from other processes. So, there is a need for heating and scrubbing light metal particles from environmental contaminants and gases in a continuous manner with no adverse environmental impact and Greenhouse Gas Emissions.